Efficient pipeline monitoring can be achieved with the help of optical based distributed sensing method and provides long term information on the pipeline integrity
As is shown in FIG. 1, usually, a fibre optic sensing cable 100 is installed in the trench 101 along the pipeline 102; once backfilled, the fibre optic sensing cable 100 lies within close vicinity of the pipeline 102.
In the configuration shown in FIG. 1, ground movement around the pipeline 102 can be measured for instance using Brillouin based (for instance BOTDA, BOTDR etc) based or Rayleigh based (for instance COTDR etc) methods. Similarly pipeline leakage can be detected and localized using distributed temperature sensing methods, for instance using Raman based or Brillouin based DTS as well as using for instance so called acoustic methods (COTDR). Third party intrusion can also be detected using for instance COTDR or interferometry based method. For DTS, the local temperature variation induced by the leak is measured and localized whilst for acoustic solution the vibration induced by the leak is measured and localized. Likewise, acoustic or interferometry solution provides information on third party intrusion. Other methods using point sensors connected by optical fibres also rely on the presence of an optical cable in the pipeline vicinity.
Often, the pipeline must cross roads, rivers, railway lines or even go below buildings. One possible approach is to use the so called Horizontal Directional Drilling method (HDD), which comprises making a small tunnel (bore hole) which is substantially horizontal to the earth surface, which is typically 1.5 times larger in diameter than the pipeline. The steps involved in a Horizontal Directional Drilling method are illustrated in FIG. 2. In this example Horizontal Directional Drilling method is performed so that a pipeline may be passed under a river 50. As shown in FIG. 2 drilling starts on one side 51 of the river 50, using a drilling rig 53 with a drill pipe 54 and drill bit 57; and a bore hole 55 is drilled and drilling is continued to reach a level which is below the river bed 56; drilling then continues horizontally under the river bed 56 before drilling towards the surface again on the opposite side 58 of the river 50.
Next a reaming of the drilled bore hole 55 is performed using a reamer 59 to provide the bore hole 55 with the desired dimension. In particular reaming is preformed to enlarge the diameter of the bore hole 55 to a desired diameter.
Once the bore hole 55 has been reamed, the pipeline 60 is pulled through the bore hole 55, from one end of the bore hole 62 to the opposite end 63 of the bore hole 55, using a pulling line 65 in a process commonly known as pipe string pullback. As shown in the figure the pulling line 65 is connected to the pipe by means of a reamer 59 and swivel 66.
It is difficult to install fibre optic sensing cables, such as those shown in FIG. 1, along pipelines which are installed in horizontal bore holes. It is necessary for the fibre optic sensing cable close to the pipe, thus inside the bore hole. However, current methods are not practical to enable this since the diameter of the bore holes are only marginally larger than the diameter of the pipeline. Thus, there is no space for a person to move inside the bore hole to install a fibre optic sensing cables along the length of the pipeline. Moreover, the bore hole is full of bentonite (fluid used to lubricate and refrigerate the drill head and reamers before and after the pull) which makes any kind of human access impossible. Furthermore due to the tight space between the pipeline and the walls of the bore hole, it is not possible, or at least very difficult, to thread a fibre optic sensing cable along the space between the pipeline and the walls of the bore hole.
A straightforward alternative is to attach the sensing cable to the pipeline, for instance on the top for convenience. However, when the pipe string pullback operation is performed, there are contact zones 105 as shown in FIG. 3, over which the pipe 106 will rub against the walls of the bore hole 108 (note that, depending on the buoyancy of the pipeline which can be positive or negative, the pipe will rub against the top or the bottom walls 109a,109b of the bore hole 108 once that the bore hole 108 is full of bentonite). If a fibre optic sensing cable is attached to the pipeline it too will rub against the walls 109a,109b of the bore hole 108. The rubbing of the fibre optic sensing cable against the walls 109a,109b of the bore hole 108 will cause damage to the fibre optic sensing cable, or may cause the fibre optic sensing cable, or parts of the fibre optic sensing cable, to become detached from the pipeline, or even broken.
Another alternative is to drill a second smaller bore hole, for the fibre optic sensing cable, adjacent to the bore hole in which the pipeline is located. However, this would increase the distance between the pipeline and fibre optic sensing cable thus reducing the sensing capacities of the fibre optic sensing cable. Moreover, an additional bore hole represents a significant additional cost to the sensing system.
It is an aim of the present invention to obviate or mitigate at least some of the above-mentioned disadvantages.